As a pollutant to the environment, chloride ions will form carcinogen chloroform when combining with the organisms in water, and chloride ions with a high concentration in water will impair the crops. Also, chloride ions having a high activity will render black metals and non-ferrous metals to be dot-erosive. Further, the sulfur-containing substances in the nature, upon the action of microorganisms, will decompose into sulfide. A large amount of sulfides are also contained in industrial wastewater from such as tannery, crude oil, paper making and chemical plants. The sulfides can cause the sensory organs to be deteriorated due to the dissipation of hydrogen sulfide in air, cause oxygen in water to be consumed and aquatic life dead.
According to Chinese National Standard (GB 11896-89), the assay of chlorides in water is carried out by silver nitrate titration method. Specifically, it is carried out as follows: in the pH range of from neutral to weak alkaline (pH 6.5-10.5), the chloride is titrated by silver nitrate with potassium chromate being the indicator. After the chloride ions are firstly completely precipitated, the chromate is then precipitated in the form of silver chromate, showing a color of brick red, which signifies the end of the titration. However, there are several problems in the method: (1) time-consuming and complicated procedures due to the manual operations in each of the steps, (2) a large consumption of chemicals, silver nitrate is harmful to the operators due to its toxicity, (3) the resistance to the interference of other ions in the assay is inferior. Additionally, there are other methods such as flow injection method (Mercuric Thiocyanate Method and Silver Ion Turbidimetric Method) and Ion Chromatography using anionic column for separation or electrical conductivity detector for detection (see Determination of Chloride Ion in Air Pollution Control Residues by Ion Chromatography, Zhangheng, etc., MODERN SCIENTIFIC INSTRUMENT, 2008(4), pages 118-119).
According to Chinese National Standard (GB 16489-1996), the assay of sulfides in water is carried out by methylene blue spectrophotometric method. Specifically, it is carried out as follows: the sample containing sulfides is firstly acidified, and the sulfide is converted into hydrogen sulfide, which is purged by nitrogen and transferred into a developer tube containing zinc acetate-sodium acetate solution, where the reaction with N,N-dimethyl-p-phenylenediamine and ammonium iron sulfate proceeds to form a blue complex methylene blue. The absorbency is determined at a wavelength of 665 nm, and the result is expressed as the apparent concentration of sulfide. However, there are several problems in the method: (1) time-consuming and complicated procedures due to the manual operations in each step, (2) a large consumption of chemicals, (3) relatively a large error for the determination and a low repeatability. In addition, on-line automatically detection methods are reported home and abroad, such as Lead Acetate Paper Strip Method, Chromatography Flame Photometric Detector Method (FPD), Ultraviolet Fluorescent Measurement and Gas Chromatography-Sulfur Chemiluminescence Detector (GC-SCD) etc.
The above mentioned manual methods and automatically analytic methods are only used to detect chlorides or sulfides, while they cannot be used to obtain the results of chlorides and sulfides in an aqueous sample simultaneously.